Wavellite from Arkansas
Contributed by: Michael Crawford
Date: Jun 17th, 2025
Locality: Montgomery County, Arkansas, USA (See on Mindat)
Size: 11 x 13 cm
Description:
This is an exceptional specimen of wavellite (Al3(PO4)2(OH)3 · 5H2O) from Montgomery County, Arkansas. The wavellite consists of radiating, acicular crystals that form \"starburst\" or \"cat’s eye\" clusters on a fracture surface of Ordovician age Bigfoot chert. The color of these starbursts transitions from the center to the outer edge, creating concentric rings within each cluster. Trace amounts of vanadium cause the green coloration of some rings, while the absence of vanadium results in clear to white rings.
I was surprised to find that the clear and white rings are brightly fluorescent bluish-white under longwave UV illumination. The fluorescence of the wavellite diminishes under midwave UV and is non-existent under shortwave UV illumination. The emission spectrum of the wavellite presents a broad peak with a maximum around 450 nm. There is also a weak white afterglow observed when the UV lights are turned off.
Wavellite is a secondary mineral that forms in near-surface environments within fractures and vugs from circulating fluids. Dissolved organic compounds in the fluids are potential activators for the bluish-white fluorescence of wavellite, while potential inorganic activators are likely to be soluble in near-surface solutions.
Summary of luminescence responses:
Wavellite
- Fluorescence under Longwave (365nm LED) UV light: White
- Fluorescence under Midwave (305nm LED) UV light: White
- Afterglow after exposure to Longwave (365nm LED) UV light: White
- Afterglow after exposure to Midwave (305nm LED) UV light: White